1. Field of the Invention
The present invention relates to a zoom lens used as, e.g., an imaging lens for a TV (television) camera, and an imaging apparatus for converting an optical image formed using the zoom lens to an electrical imaging signal.
2. Description of the Related Art
In an optical system using a solid-state imaging device such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), since these devices have high sensitivity even in an infrared region, an infrared ray cut filter reducing a transmittance near the A′-line (about 750 nm) to about 50% (half value) is often inserted. Nevertheless, since aberration in a red region may significantly affect image quality and resolving power, correction of chromatic aberration from the C-line (656.3 nm) to the A′-line (768.2 nm) is important. In addition, in the optical system having moving groups, the chromatic aberration may be corrected for each group such that the chromatic aberration is not fluctuated during variation of magnification. Relationally, in the correction of the chromatic aberration, particularly in the correction of secondary spectrum, it is known that the use of an optical material having an anomalous dispersion characteristic is effective (see JP-A-2007-163964 corresponding to U.S. Pat. No. 7,292,398 B1, JP-A-2006-349947 corresponding to U.S. Pat. No. 7,304,805 B2, JP-A-2005-345892 corresponding to U.S. Pat. No. 7,139,131 B2, and JP-A-2006-78964).
However, with attention focused on the correction of the chromatic aberration in a blue region, only a lens system paying attention to a partial dispersion ratio in a blue region, e.g., the partial dispersion ratio θg,F related to the g-line (435.8 nm) and the F-line (486.1 nm) is described, and the correction of the chromatic aberration in the red region is not sufficient. In particular, there is a case where the blue region and the red region have different tendencies of the anomalous dispersion characteristic depending on materials, and therefore, there is a possibility that the sufficient correction of the chromatic aberration may not be performed with attention paid only to the anomalous dispersion characteristic in the blue region.